Mapper class names in registry should not be required to be globally unique

[jackqu7]

For example, if your application includes third party libraries that define their own Kim Mappers, you're likely to have conflicts with common names such as User.

A simple workaround could be to namespace them by module, using sensible defaults to mean in most cases it's not required to use the full path.

ie. If the Nested is in foo.serializers, look there first rather than bar.serializers

[jackqu7]

(don't forget to update the docs, which currently say it's not possible)